Method for simulating a controlled voltage for testing circuits for electromagnetic susceptibility

ABSTRACT

Apparatus and method for low cost monitoring the level of signal at a test point in a system for susceptibility to electromagnetic fields. A probe, including a detector diode, and a non-metallic, electrically overdamped conductor, which is transparent to the electromagnetic field, is used to monitor the signal level at a test point as an amplitude modulated radio frequency carrier. The carrier is transmitted to a monitor outside of the range of the electromagnetic field using a transmission link, such as an optical waveguide transmitter, that is transparent to the electromagnetic field when the system under test fails. The system under test can then be removed from the electromagnetic field and, for each frequency at which the system failed, a voltage can be injected, using a voltage injection probe, into the system at another point to recreate the detected level of signal at the test point that was coupled into the system from the electromagnetic field. This simulates the effect of the susceptibility to the electromagnetic field, and permits testing of a suitable filter or other expedient applied to the system, even though the system is not exposed to the electromagnetic field. Thus, the system may be tested for susceptibility inside a shielded enclosure and subjected to a controlled electromagnetic field, and the susceptibility may be recreated and solved outside of the shielded enclosure. The probe may include a plurality of detector diodes mounted on a printed circuit board in a shielded structure that is directly connected to the test wire of the circuit to be monitored.

This is a divisional of U.S. patent application Ser. No. 08/388,194,filed on Feb. 13, 1995 and entitled Apparatus for Low CostElectromagnetic Field Susceptibility Testing, now U.S. Pat. No.5,552,715, which is a divisional of U.S. patent application Ser. No.08/044,219 filed Apr. 7, 1993 and entitled Apparatus and Method forLow-cost Electromagnetic Field Susceptibility Testing, now U.S. Pat. No.5,414,345, which is a continuation-in-part of U.S. patent applicationSer. No. 07/692,719, filed Apr. 29, 1991 and entitled ElectromagneticField Susceptibility Test Apparatus and Methods, now abandoned.

FIELD OF THE INVENTION

The present invention relates to the field of testing the susceptibilityof devices and systems to radiated electromagnetic fields, and E fieldsin particular.

BACKGROUND OF THE INVENTION

Analog and digital electronic circuitry and attendant wiring mayencounter serious operating difficulty in the presence of radiatedelectromagnetic fields. When such fields interfere with the operation ofelectronic circuitry, they are generally referred to as ElectromagneticInterference (EMI) fields. The circuits and attendant wiring may beshielded and filtered to provide some immunity to large electromagneticfields, however, it is not possible or practical to design the circuitryand attendant wiring so as to ensure immunity to such fields. Indeed,shielded cables can sometimes be more susceptible to EMI problems thanunshielded wires, because bending of a shielding cable, or other abuse,may produce a pin hole in the shielding, which can cause the shieldingto act as a waveguide for EMI, with consequent deleterious effects.

Methods and apparatus, therefore, are required to test thesusceptibility of the devices, such as integrated circuits, electricalcomponents, and the like, and systems, such as automotive electricalsystems, for both commercial and military applications, toelectromagnetic fields. The term "system" is employed hereinafter tobroadly describe any device or system, such as, but not limited to thosedescribed above, that may be tested for susceptibility to radiatedelectromagnetic fields.

Electromagnetic field testing is typically performed in shieldedenclosures, or "screen rooms," which provide an environment whereinambient electromagnetic fields are eliminated and a controlled field isproduced, in order to determine, with certainty, the effect of a givenlevel (measured in volts per meter) and frequency of electromagneticfields on the system undergoing test. Apparatus typically used insidethe shielded enclosure includes current probes attached to a systemharness wire and a transmitter which sends the signals detected by theprobes to a receiver outside the shielded enclosure, where the effectsof the electromagnetic fields on the system are determined.

To ensure the integrity of the shielded enclosure and the results of thetests, any voltage measuring apparatus within the screen room shouldminimally perturb the controlled electromagnetic fields and should notinject EMI into the system. For example, any test apparatus which mightreradiate electromagnetic fields impinging on the device under test, ormight itself be susceptible to such fields or otherwise inject any noiseinto the system, must be avoided.

In U.S. Pat. No. 4,939,446, by Wesley A. Rogers, the inventor of thepresent invention, the performance of a system under test is observed ina shielded enclosure, with and without the presence of controlled Efields radiated by one or more antennas. The problem of reradiation offields, or the injection of noise into the system by the test equipmentitself, is eliminated through the use of non-metallic overdampedconductors, and a hybrid electrical/optical transmitter and opticalcable used to transmit voltage signals from the system to a receivermonitored by an oscilloscope located outside the shielded enclosure. Theapproach set forth in the above patent allows accurate testing ofsystems in a controlled electromagnetic environment, since theoverdamped conductors are transparent to the electromagnetic fields. Theentire disclosure of U.S. Pat. No. 4,939,446 is incorporated herein byreference thereto.

The standard operating procedure in determining the susceptibility ofsystems to radiated electromagnetic fields is to place the system in ashielded enclosure, as mentioned above, and to test the susceptibilityof the system over a wide range of radio frequencies, for example from10 KHz to 18 GHz or more. The radiated field is swept through thedesired frequency range, at a range of levels, e.g. between 1 v/m and300 v/m, and the susceptibility of the system is determined over therange of frequencies. If, for example, it is determined that the systemis susceptible to a radiated field at 2 GHz and a certain volt per meterlevel, a suitable filter or other expedient can be placed in the system,and the system retested at that frequency and level to see whether thefilter is effective in removing the system's susceptibility. If thesystem is still susceptible, the filter must be changed, or anotherapproach must be adopted.

This trial and error technique must be used for each frequency and levelat which there is a susceptibility problem. Moreover, the trial anderror testing and retesting has to take place in a shielded enclosure,as referred to above, since the FCC prohibits the generation of theantenna-radiated fields necessary to conduct open air testing, and sincethere is no way to tell what effect, if any, ambient fields might haveon the system--the controlled environment provided by the shieldedenclosure is necessary to accurately determine susceptibility problems.The testing and retesting of systems, in order to troubleshoot andalleviate susceptibility to radiated fields, usually requires betweenone and two weeks for each system. The rental of a shielded enclosure,such as a screen room, can cost between $1,000-$3,000 a day, and thusruns into considerable expense, and constitutes a bottleneck for thedevelopment of new systems. Furthermore, even with this procedure, thereis no indication of the voltage level of EMI induced in the system thatcaused a failure. A test technique that allows an accurate determinationof that level has long been desired.

The RF Probe described in the prior U.S. patent application Ser. No.08/153,502, now U.S. Pat. No. 5,414,366, is particularly useful forfrequencies in the range of 10 KHz to 18 GHz or more, which areparticularly appropriate for military applications. However, it is nowrealized that susceptibility testing at the higher frequencies, namelyfrom 1.4-18.0 GHz, are not necessary for use in most commercialapplications. Accordingly, it is desirable to obtain a lower cost RFprobe that operates in the range of from 10 KHz to on the order of 1.40GHz.

OBJECTS AND SUMMARY OF THE INVENTION

It is therefore an object of the present invention to overcome thedifficulties associated with the prior art.

It is a further object of the present invention to allow rapid and lessexpensive testing of systems for susceptibility to radiatedelectromagnetic fields.

It is a further object of the present invention to provide a probe thatallows the level of a radiated field coupled into a system to bemeasured, without injecting the radiated field energy into the system,or otherwise affecting the level of EMI coupled into a system.

It is a further object of the present invention to provide an apparatusand method for accurately correcting for susceptibility to radiatedfields outside of a shielded enclosure--after initial testing in ashielded enclosure.

It is a further object of the present invention to provide an apparatusand method for determining the level of EMI coupled into a system forparticular levels and frequencies of radiated fields.

It is a further object of the present invention to provide apparatus forradio frequency (RF) voltage injection into a wire of a system undertest.

It is a further object of the present invention to provide an apparatusand method that allow the injection of RF voltages into a system at alevel and frequency so as to accurately simulate the susceptibility ofthe system to particular levels and frequencies of radiated fields.

It is a further object of the present invention to provide apparatus andmethods for testing a device under test for susceptibility to radiatedelectromagnetic fields at a plurality of wires using a correspondingplurality of probes. It is another object to multiplex the output of theprobes to the outside of the shielded enclosure for monitoringselectively, one or more of the test wires to which each probe isattached, external of the shielded enclosure.

In accordance with the present invention, a probe is first used tomonitor the level of a radiated electromagnetic field, in the form of anamplitude modulated (AM) RF carrier, coupled into a system in a shieldedenclosure. The probe includes a detector diode which is placed at aselected part of the system. The output of the detector diode, isapplied to a monitor outside the shielded enclosure through the use of anon-metallic, overdamped wire, and a suitable transmission link. Theprobe and non-metallic wire are neither susceptible to the radiatedfield, nor do they inject EMI into the system under test.

The amplitude modulation detected by the detector diode is easily withinthe pass band of the non-metallic, overdamped wire so that the detectedsignal can pass to the monitor, through the overdamped wire and throughthe transmission link, which can be in the form of an opticaltransmitter.

In accordance with the present invention, the probe also may be used tomonitor the level of a radiated electromagnetic field in the form of acontinuous wave RF carrier coupled into a system in the shieldedenclosure. The probe includes a detector diode which is placed at aselected part of the system. The output of the detector diode, which isa DC signal, is applied to a monitor outside the shielded enclosurethrough the use of a non-metallic, overdamped wire, and a suitabletransmission link. The probe and non-metallic wire are neithersusceptible to the radiated field, nor do they inject EMI into thesystem under test.

The radiated signal detected by the detector diode is easily within thepassband of the non-metallic, overdamped wire. The detector diode mayfurther comprise a plurality of detector diodes, e.g., when the peak topeak voltage level of the radiated signal is greater than the reversebias voltage of each detector diode. In such case, additional detectordiodes are used in series to distribute the voltage at the probe inputacross the detector diodes. The detector diode or diodes are preferablymounted on a conventional printed circuit board having a ground planewhich is connected to a copper shield. Both the copper ground plane andcopper shield are allowed to float. This detector diode is suitable fordetecting susceptibility at frequencies up to about 1.4 GHz. Thedetector diode also may be operated with the shield connected to ground,unless undesirable resonances occur in the grounded condition. In suchcase, the shield should be allowed to float.

The voltage level and frequency of EMI coupled into the system, asdetected by the detector diode, is noted whenever the system under testfails. The system can then be removed from the shielded enclosure andfor each frequency and level at which the system was susceptible to thefield, an amplitude modulated RF voltage or a continuous wave RF voltagecan be injected into the system at a level sufficient to recreate thelevel of EMI coupled into the system in the shielded enclosure, therebyessentially replicating the effect of the controlled field level in theshielded enclosure. A suitable filter or other expedient can then beapplied to the system, the same voltage level can be injected into thesystem, and the system can be observed to see whether it no longer failsunder such conditions.

In accordance with another aspect of the present invention, the probecomprises a microwave circuit designed to maintain the input impedanceof the diode constant throughout the frequency range of interest.

In accordance with another aspect of the present invention, a capacitivevoltage injection probe is provided for allowing the injection ofvoltage into the system under test when in accordance with the testingprocedure described above.

In accordance with another aspect of the present invention, a pluralityof probes are connected to a plurality of wires in the device under testand the output of the probes are connected to a multiplexor deviceinside the shielded enclosure. The multiplexor device provides an outputthat can be passed out of the shielded enclosure to a correspondingdemultiplexor. The demultiplexor then recreates the probe output signalsfor display or printing, e.g., on a strip chart. Themultiplexor-demultiplexor system may use a control circuit forcontrolling the multiplexor to sequence through-the plurality of inputs.Preferably, a time multiplexing arrangement is used whereby each of theprobes is sampled periodically and the signals at each of the test wirescan be displayed external of the shielded enclosure. A frequencymultiplexed arrangement also may be used whereby the output of eachprobe is converted into a unique frequency signal such that uniquefrequencies are collectively transmitted to a demultiplexor external tothe shielded enclosure over a common conductor, e.g., optical fiber, andthen separated for separate display. The latter technique provides for acontinuous display of each test wire being monitored and is simpler withrespect to avoiding the requirement for synchronizing the clock in thetime based multiplexor.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects, aspects and embodiments of the invention willbe described in more detail with reference to the following drawingfigures of which:

FIG. 1 is a diagram illustrating the use of the probe, non-metallicconductors and transmission link, in accordance with one aspect of thepresent invention, in a shielded enclosure, to determine the level of afield coupled onto wires of systems under test;

FIG. 1A is a diagram illustrating the use of a low cost RF probenon-metallic conductors and transmission link, in accordance with oneaspect of the present invention, in a shielded enclosure, to determinethe level of a field coupled onto wires of systems under test;

FIGS. 2A, 2B, 2C, 2D, 2E, 2C¹, 2D¹, 2F and 2G depict signals radiated,coupled, detected and injected, in accordance with the method ofdetermining and correcting the susceptibility of systems to EMI, inaccordance with another aspect of the present invention;

FIG. 3 is a diagram illustrating the use of the probe, non-metallicconductors and transmission link outside the shielded enclosure, tosimulate the conditions within the shielded enclosure.

FIG. 4 is a schematic diagram illustrating a specific embodiment of thedetector probe circuitry used in accordance with the present invention;

FIGS. 5A and 5B illustrate an example of the microwave circuit boardimplementation of the circuitry illustrated in FIG. 4;

FIG. 6 is a diagram illustrating the detail of the connectors employedin the microwave circuit board implementation of FIGS. 5A and 5B;

FIG. 7 is a schematic illustration of circuitry employed in voltageinjection, in accordance with another aspect of the present invention;

FIG. 8 is a schematic diagram illustrating a specific embodiment of thelow cost detector probe circuitry used in accordance with an embodimentof the present invention;

FIGS. 9A and 9B illustrate an example of the printed circuit boardimplementation of the circuitry illustrated in FIG. 8; and

FIGS. 10, 11A and 11B, 12, 12A and 12B are respectively four differentembodiments of a multiplexor-demultiplexor system for monitoring aplurality of wires of a device under test using the probe of FIG. 8.

DETAILED DESCRIPTION OF THE INVENTION

With reference to FIG. 1, a probe 1, in accordance with one aspect ofthe present invention, is illustrated along with an optical transmissionlink, as used in accordance with the method of determining andcorrecting for EMI susceptibility of a system under test, in accordancewith another aspect of the present invention. Specifically, the probe 1is connected to a lead 10 of the system under test, such as anautomotive electrical system, byway of a clip or pin 12 which can eithercomprise a short conductive clip or pin, having a length less than orequal to one centimeter, to prevent the field from coupling onto theclip or pin. Alternatively, a longer clip or pin can be employed, havingits insulation coated with a conductive or reflective paint or foil,open at both ends, to effectively shield it from EMI.

The detector probe 1 is also connected to a ground wire 18 from thesystem under test by way of non-metallic conductor 22, and to opticaltransmitter 2, by way of nonmetallic conductors 20 and 50. Thenon-metallic conductors 20, 22 and 50 can be formed of one or morecontinuous monofilament cores of plastic which are impregnated with fineconductive particles, such as carbon, and covered with the plasticinsulating sheet. A conductor material with the above describedcharacteristics is marketed by the Polymer Corporation of Reading, Pa.under the trade name FLUOROSINT® 719. It comprises a carbon/fluropolymercore 0.030 inches in diameter enclosed by a transparent nylon insulatingcover with an outer diameter of 0.040 inches and has resistance perlength of cable within the range of 20,000 to 30,000 ohms per foot. Suchnon-metallic cores have a uniformly distributed resistance, capacitanceand inductance, causing them to be electrically overdamped andtransparent to electromagnetic fields. It must be noted, however, thatthe bandwidth of such cores is usually no more than approximately 20MHz, much less than many of the frequency ranges of interest.Preferably, the conductors 20, 22 and 50 can be-formed of four or fivesuch monofilament cores to reduce the linear resistance to about 6Kohms/foot, the conductors 20, 22 and 50 preferably providing a totalresistance on the order of 10,000 to 20,000 ohms. Optionally, withreference to FIGS. 1 and 3, optical transmitter 2 can be directlyconnected to ground 18 of the device under test by a non-metallicconductor such that conductors 50 and 22 may be eliminated (not shown).

It will be appreciated, in view of the above mentioned U.S. Pat. No.4,939,446, that by maintaining proper lengths of the metallic conductorssuch as pin 12, by covering the insulation of such elements withconductive paint or foil, and by using the non-metallic conductors 20,22 and 50, the probe in accordance with the present invention istransparent to electromagnetic fields, will not re-radiate such fields,and will not inject EMI into the system under test.

The other ends of non-metallic conductors 20 and 50 are applied to anoptical transmitter 2, which includes a high input impedance amplifierhaving a gain on the order of approximately 1,000, although the gain ofthe amplifier can be selected as desired, and an LED, driven by theamplifier, which delivers a light signal to optical cable 26. The exactform of the amplifier and associated circuitry is not critical and canbe comprised of a standard low drift DC or analog amplifier, but stateof the art techniques should be used to insure minimum operationalamplifier offset voltages. The opposite end of the optical cable 26 isapplied to an optical receiver 3, which can include a photo diode orother suitable light detector, and an amplifier which receives theoutput of the photo diode, and which delivers an output that can beviewed on a voltage monitor 4 or a plurality of monitors, if desired.The monitor 4 can be any voltage sensitive device capable of detectingsignals down to the microvolt level, such as an oscilloscope or spectrumanalyzer.

The detector probe 1 and optical transmission link can be used todetermine the level of a radiated field coupled into the system undertest that causes the system to fail. As shown in FIG. 1, the systemunder test, the probe 1, the transmitter 2 and a portion of the opticalcable 26, are disposed within a shielded enclosure 5 which provides acontrolled electromagnetic field environment, through the activation ofone or more antennas 6 which radiate an "E" field. The antenna 6 isdriven by RF generator 7 in a well-known manner and is connected theretoby way of shielded cable 8. As mentioned above, shielded enclosures,such as screen rooms, are routinely used in testing systems forelectromagnetic field susceptibility since FCC regulations prohibit theuse of such radiated fields in the open air, and because systems beingtested outside the controlled environment of the enclosure might beaffected by ambient fields, and a precise cause-and-effect relationshipcould not be determined. By testing within the shielded enclosure,ambient fields are eliminated and the field level applied to the systemis carefully controlled.

The optical cable 26 and shielded cable 8 pass through the enclosure 5by way of waveguide-beyond-cutoff filter 36 and coaxial feedthroughconnector 37, respectively. The optical transmitter 2 should be disposedwithin an RF shielded enclosure, but alternatively, the non-metallicconductors 20 and 50 may extend out through the filter 36, in which casethe optical portion of the transmission link can be omitted, the outputsof conductors 20 and 50 being passed directly to an amplifier outsidethe enclosure 5.

In order to determine the level of EMI coupled into the system undertest that causes the system to fail, RF generator 7 is activated tocause antenna 6 to radiate an E field in the form of an amplitudemodulated carrier at a particular frequency, for example, a 1 GHzcarrier, 100% amplitude modulated by a 1 KHz signal, but thefrequencies, modulation levels, etc., as well as the radiated fieldlevel, can be chosen as desired, in a manner well known in the art. Thesignal radiated from antenna 6 is illustrated in FIGS. 1 and 2 as signalA.

A certain level of the radiated field will be coupled into the system toproduce a coupled signal voltage B, as shown in FIGS. 1 and 2. The levelof the coupled signal B is detected by the detector probe 1 and theenvelope, corresponding to the detected signal C, as shown in FIGS. 1and 2, is produced at the output of the detector probe. In the abovedescribed example, signal C is at the modulation frequency of 1 KHz,which is easily transmitted along the restricted bandwidth of thenon-metallic conductors 20 and 50. The signal C is applied to opticaltransmitter 2, and it, or a signal related to its average, RMS, or peakvoltage, such as voltage level D, FIG. 2, is transmitted to the opticalreceiver 3, and monitored by monitor 4. Conversion of signal C to arelated voltage level, such as the D.C. average voltage, voltage levelD, can be performed before or after transmission, and if done aftertransmission, a 1 KHz notch filter, in this example, can be used in thetransmission path in order to eliminate any DC offsets and/or noise onthe non-metallic wire or system under test. Alternatively, the signal Ccan be transmitted and monitored directly at monitor 4.

In an alternate embodiment, the level of EMI coupled into the systemunder test that causes the system to fail can be determined in responseto a radiated E field in the form of a continuous wave carrier at aparticular frequency, for example, a 10 GHz carrier, having apeak-to-peak voltage as illustrated in signal E of FIG. 1A and FIG. 2F.A certain level of the radiated field will be coupled into the system toproduce a coupled signal G having a DC level bias, as shown in FIGS. 1and 2G. The level of the coupled signal is detected by the detectorprobe 1, corresponding to the DC level indicated in signal C, as shownin FIGS. 1 and 2, and is produced at the output of detector probe 1. Inthis example, signal C is a DC level which is easily transmitted alongthe restricted bandwidth of the non-metallic conductors 20 and 50. TheDC signal G is applied to optical transmitter 2 and it, or a signalrelated to its value, such as the signal H, is transmitted to theoptical receiver 3 and monitored by monitor 4.

As is well known in EMI testing, a power supply is typically employed tosupply electrical power to the system under test, through a LineImpedance Stabilization Network (LISN). LISN's are used in most EMI testset-ups to stabilize the test against variations in the impedance of thepower supply.

As the testing proceeds, the carrier frequency, whether or notmodulated, and volt/meter level of the radiated E field, produced byantenna 6, is swept in frequency (usually from 10 KHz to 18 GHz), and inlevel until the system under test fails, at which point, the voltage D(or H) detected at the monitor 4 (FIGS. 1 and 2) and the carrierfrequency are noted, and the frequency and signal level are swept againand the process repeated. The result of such testing will be a table ofradiated field levels and frequencies, at which the system failed, alongwith the corresponding voltage levels D detected by the detector probe1.

The system under test, the detector probe 1, the nonmetallic conductors20, 22 and 50, and the optical transmitter 2 can then be moved out ofthe enclosure 5 as shown in FIG. 3. A voltage injection probe 90, drivenby a standard RF generator 91, is connected to wires 10 and 18 of thesystem under test, as shown. RF generator 91 must employ the samemodulation index (in this example 100%) as the RF generator 7 used inthe screen room. Then, at each frequency at which the system failed, anamplitude modulated signal (signal E, FIGS. 2 and 3) is injected intothe system using the voltage injection probe 90 to be described indetail with reference to FIG. 7, below. The modulation on the injectedsignal E is detected by the detector probe 1, and the voltage level ismonitored by monitor 4, as discussed in connection with FIG. 1. Thelevel of the injected signal (signal E) is then adjusted at generator 91until the voltage level D' measured at monitor 4, is equal to the levelD for that frequency measured in the shielded enclosure. When this levelis reached, the controlled conditions of the shielded enclosure areessentially replicated outside of the enclosure. An EMI filter or otherexpedient can then be placed in an appropriate location in the systemunder test, and the signal at the level determined as discussed above,can be injected again into the system under test, to see whether thesystem still fails under such conditions. If it still fails, a differentapproach, such as a different filter, can be employed, and the processrepeated until the system no longer fails. The generator 91 can then beadjusted to the next frequency (if any) at which the system failed inthe shielded enclosure, and the above procedure repeated. It should beunderstood that the foregoing applies to a signal (signal F, FIG. 1)which is not amplitude modulated, whereby the detected signal inresponse to the injected signal is the same outside the enclosure as itwas inside the enclosure for the carrier frequency at which failureoccurred.

A specific embodiment of the circuitry employed in detector probe 1 inaccordance with another aspect of the present invention, mounted on amicrowave circuit board 40, will now be described with reference to FIG.4. The clip or pin 12 (FIGS. 1 and 4) is connected to the anode of azero bias Schottky detector diode 14, such as the HSCH-3486 leadedpackage or HSCH-3207 microstrip package, available from Hewlett-Packard,but other detector diodes can be used, depending upon desired frequencyranges, bandwidths and the like. The diode leads should be maintained asshort as possible and may be painted with a reflective paint to minimizecoupling of radiated fields into the diode leads. Preferably, themicrostrip package diode leads should be used. The diode should bemounted using microstrip or mounting techniques for maintaining inputimpedance substantially constant over the frequency range of interest.One example of such a technique will be described with reference toFIGS. 5A-5B, below. Although the use of a zero bias Schottky detectordiode is preferred, non-zero bias diodes can be used but requirebatteries within the shielded probe to provide an appropriate DC bias.

The anode of diode 14 is connected to pin 12, for insertion into lead 10of the system under test, by conductor 38, and the cathode of diode 14is connected to non-metallic conductor 20 by way of conductor 39 andconnector 42. The cathode of diode 14 is also connected to non-metallicwire 22 by way of capacitor 44, resistor 46, connected in parallel withcapacitor 44, conductors 39 and 48 and connector 43, which connects theconductor 48 and non-metallic conductor 22. Conductor 48 is alsoconnected to non-metallic conductor 50 by way of Connector 52. Thenon-metallic conductors 20 and 50 are connected to an amplifier in theoptical transmitter 2, such as the amplifier disclosed in FIG. 2 of U.S.Pat. No. 4,939,446, although the particular form of the amplifier is notcritical, in which case non-metallic conductor 20 is applied to the "A"input terminal, and non-metallic conductor 50, is connected to the "G"(ground) terminal of the amplifier, the B terminal remaining unattached.Finally, the anode of diode 14 and the conductor 48 are connected toeach other by way of parallel connected resistors 54a and 54b.

In operation, the capacitor 44 charges during conduction of diode 14,and the resistor 46 allows capacitor 44 to repetitively discharge asrequired to produce the detected envelope. The resistors 54a and 54bstabilize the input impedance to the diode over the entire input signalbandwidth, thus reducing the standing wave ratio at the diode toapproximately 1.25.

With reference to FIGS. 5A and 5B, a specific implementation of themicrowave circuit board 40 will be described, the same referencenumerals being used in FIGS. 1, 3, 4, 5A, 5B, 6 and 7 to indicatecorresponding elements. Pin 12 is mounted on conductor 38 which isformed of a 57-ohm microstrip trace. Diode 14 is mounted using standardmicrowave mounting techniques to the 57 ohm microstrip trade 38 at itsanode, and at its cathode to conductor 39 which is formed of a copperconductor flange. The flange 39 flares outwardly from the diode asshown, to form a generally triangular shape, which reduces theinductance of the current path between the cathode of the diode and theconnector 42 at high frequencies. Conductor 48 is formed of a copperground plane, underneath a printed circuit board substrate 56 whichelectrically separates the conductor 48 from the conductor 38 and thecopper flange 39. The ground plane immediately below the diode can beetched away to reduce the effect of diode chip parasitic capacitance andachieve the desired frequency response. The parallel resistors 54a and54b are formed of two 114 ohm microstrip resistors which are depositedby thin film deposition techniques on the 57 ohm microstrip trace 38 andsubstrate 56, and are connected to the conductor 48 by way of gold foilwraparounds 58a and 58b.

Capacitor 44 is a feed-through capacitor of the flat tubular discoidaltype, the outer case thereof being attached to conductor 48 by eithersoldering or bolting methods compatible with RF circuit designtechniques. The resistor 46 is formed of a coil of non-metallic wirehaving a resistance of about 10K ohms, connected at its ends toconnectors 42 and 52, as will be described, but alternatively, aresistor having a 10K ohm impedance can be employed. Alternately, thecoil resistor 46 could be omitted.

Connector 42 is connected to flange 39 by way of copper block 60, whichis soldered to the flange, connectors 43 and 52 are connected to theground plane conductor 48 by way of copper blocks 62 and 64,respectively, soldered thereto, and pin 12 is connected to conductor 38by way of copper block 66 similarly soldered thereto. Pin 12 should beless than about one centimeter in length and can be formed of anysuitable sharp conductor, such as a portion of a needle, or the like.

The construction of the connectors 42, 43 and 52 will now be describedin more detail with reference to FIG. 6. A male pin 68, such as theDupont/Berg P/N 48116-000 is inserted into a plastic housing 70, such asthe Dupont/Berg P/N 65039-036, to give rigidity to the pin 68, and thepin and housing are connected to the circuit board by way of arespective copper block 60, 62 or 64. The female receptacle 72, such asthe Dupont/Berg P/N 47745-000, is inserted into an associated plastichousing 74, the same as or similar to housing 70, to add rigidity to thereceptacle 72. A conductor 76, such as an 18-gage pin, is soldered toreceptacle 72 and inserted into one end of a metallic sleeve 78. One ofthe non-metallic conductors 20, 22 or 50 is inserted into the other endof metallic sleeve 78 so that it overlaps conductor 76, and the metallicsleeve is crimped at 80 in order to conductively connect the conductor76 to the non-metallic conductor. In the case of connectors 42 and 52,the respective ends of the resistive coil 46 are inserted into themetallic sleeves 78 along with non-metallic conductors 20 and 50, asshown in FIG. 5B.

Returning to FIGS. 5A and 5B, although not shown to scale, the circuitboard 40 will be approximately one-half inch wide, one inch long andone-quarter inch deep. The circuit trace lengths, depths and widths canbe determined by those skilled in the art through standard RF designanalysis. The circuit board, along with the components thereon, ispreferably disposed within a standard RF shielded oscilloscope typeprobe assembly. Preferably, the connectors 42, 43 and 52 are alsodisposed within the shielded probe assembly, but if not they should beseparately shielded.

As mentioned above, the non-metallic conductor 20 can be attached to theA input of the amplifier disclosed in the above referenced patent, andnon-metallic conductor 50 can be applied to the ground input thereof.Both conductors 20 and 50 can be connected to the amplifier by way ofstandard shielded BNC connectors 82 and 84, respectively.

The needle 12 is inserted into a system wire 10, and the non-metallicconductor 22 is provided with a needle or pin 86 which is inserted intoone end of an associated metallic sleeve 88, the other end of whichreceives conductor 22. The metallic sleeve 88 is crimped, in a mannersimilar to that shown in FIG. 6 with respect to metallic sleeve 78, inorder to electrically connect the pin 86 to the conductor 22. The pin 86is inserted into system ground wire 18. Optionally, with respect to FIG.5B, the ground input may be directly connected to system ground wire 18by a non-metallic conductor (not shown) and pin 86 such that conductors22 and 50 and connectors 43 and 52 are eliminated and ground plane 48 isallowed to float.

In accordance with another aspect of the present invention, the RFvoltage injection probe 90 will be described with reference to FIG. 7.The injection probe is used to inject radio frequency voltages from astandard signal generator (generator 91, FIG. 3) into the system undertest in accordance with the test procedures described above. Theinjection probe is housed in a metallic RF shielded housing 92 and isconnected at its input to a standard signal generator, for generatingapproximately 10 KHz to 18 GHz, amplitude modulated signals by way ofcoaxial cable 94. The output of the injection probe is connected towires 10 and 18 of the system under test, by coaxial cable 96, the cableshielding being connected to the housing 92 through the use of standardBNC connectors. Coaxial cable such as that designated RG 58 can beemployed, and standard BNC connections can be employed for connection ofthe cables to the injection probe 90. The connection between the probe90 and the system wires 10 and 18 can be made with the use of test clipssuch as those provided by ITT Pomona Electronics, Model Nos. 5188 or3788, for example. The BNC connectors carry the shield as a groundreturn.

The injection probe 90 is comprised of five parallel connectedcapacitors, in this example. Specifically, the 0.47 microfaradcapacitors are, in this example, paper type capacitors which function topass signals between approximately 10 KHz and 100 KHz. The 0.1microfarad. capacitor is, in this example, a plastic type capacitordesigned to pass frequencies between about 100 KHz and 20 MHz, and the0.01 and 0.001 microfarad capacitors are, in this example, of the silvermica type that pass signal frequencies between about 20 MHz and 150 MHz.The coaxial connectors on the shielded housing 92 should incorporatecoaxial feedthrough connectors that pass frequencies above approximately150 MHz.

Injection probe 90 thus comprises a number of parallel capacitive pathsfor injecting voltages over the desired frequency range, each capacitivepath handling a particular frequency range. By employing the injectionprobe 90, the desired level of voltage for producing voltage level D',in FIG. 2, can be readily achieved using a standard signal generator,without requiring expensive bulk current injectors.

The injection probe frequency response characteristics are automaticallycompensated for since the voltage output from generator 91 (FIG. 3) isadjusted until the correct voltage level is injected into system, asdetermined by monitoring the detected output, as discussed above.

With reference to FIGS. 8, 9a, and 9b, a specific embodiment ofcircuitry employed in detector probe 1 in accordance with yet anotheraspect of the present invention, is shown. This implementation will bedescribed using the same reference numerals that are used in FIGS. 1, 3,4, 5a, 5b, 6 and 7 to indicate corresponding elements, to the extentappropriate. In this embodiment, the detector probe 40' is fabricatedusing conventional circuit board construction techniques and is suitablefor use for the narrower bandwidth of from 10 KHz to 1.4 GHz.

Referring to FIG. 8, the clip or pin 12 is mounted on printed circuitboard 40' and connected to the anode of a zero bias Schottky detectordiode 14a, as described above. The cathode of detector diode 14 isconnected to connector 42. A second probe 48 is connected between theground input 84 of the instrumentation and the ground referenced 18 ofthe device under test for the wire 10 that is being monitored. Printedcircuit board 40' also includes a ground plane 97 (not shown in FIG. 8)which is allowed to float. Alternately, ground plane 97 and conductor 48may be the same element, connected in the manner illustrated in FIG. 5B.In the embodiment shown in FIG. 8, a series resistor 54c is interposedbetween pin 12 and the anode of diode 14a. This resistor 54c workstogether with diode load resistor 54d, which is placed in parallelacross the probe conductor 20 and conductor 48 outputs to provide forinput voltage attenuation, if any is required. Resistor 54c may be anarbitrary series microstrip resistor. Resistor 54d may be a highresistance such as 100 KΩ to 1 MΩ resistor which is provided to improvethe performance of diode 14a. With this construction, resistors 54c and54d form a voltage divider that can be selected to allow use of a singlediode 14a to monitor high RF voltage levels, peak-to-peak, in wire 10 ofthe device under test.

In a preferred embodiment, resistors 54c and resistor 54d are omittedand instead a string of Schottky diodes, placed in series, are used. Theconventional Schottky diode cannot withstand more than an 4 volt reversebias signal. Consequently, when monitoring RF input voltage greater than8 volts peak-to-peak and a voltage divider is not used, additionalseries diodes are required to distribute the voltage drop. For example,ten series diodes are required to withstand an 80 volt peak-to-peaksignal at the probe input. In addition, it was discovered that using aconventional series resistor mounted on a conventional printed circuitboard (i.e., not a microstrip printed circuit board) will allowfrequencies above 29 MHz to blow by the resistor without anyattenuation. This was discovered not to occur with the use of aplurality of series Schottky diodes. Accordingly, in the preferredembodiment, the lower bandwidth circuit shown in FIG. 8 typicallyincludes an arbitrary number of series diodes, of which five areillustrated as diodes 14a, 14b, 14c, 14d, and 14e, mounted on aconventional printed circuit board 40'.

The circuit board layout is shown in FIGS. 9A and 9B. Ground plane 97 istypically oriented on its underside and is connected to a copper shield99 by a lead 98. This is shown in FIG. 9b. As noted, the copper groundplane 97 and copper shield 99 are generally allowed to float, but may begrounded, whereupon if resonance occurs, then the shield 99 andgroundplane 97 should be allowed to float.

The number of Schottky diodes 14 that can be placed in series on anon-striped line type printed circuit board is limited by the dimensionconsiderations with respect to the excitation frequency and resonance.For example, the maximum length of the printed circuit board should notbe greater than about 2 centimeters when operating at 1.4 GHz. In theevent that a stripline printed circuit board is used, this limitationdoes not apply and any number of diodes may be used. Accordingly, asillustrated in FIGS. 9a and 9b, diodes 14a-14e are illustrated asmounted side by side in order to minimize the length of the printedcircuit board. In fact, the diodes may be mounted in touching contactside to side, with the anode and cathode leads as short as possible, aspreviously described. The printed circuit board traces are illustratedin FIGS. 9a and 9b by the reference 55. It is noted that 24 Schottkydiodes may be mounted on circuit board 40' although only five suchdiodes are shown. Copper shield 99 envelops circuit board 40' inelectromagnetic isolation from the circuit path between pins 12 and 60.

One of the advantages of the probe illustrated in FIGS. 8, 9a, and 9b isthat it has a substantially lower cost than the RF probe illustrated inFIGS. 4, 5a, and 5b. This is in part due to the bandwidth limitation ofthe RF probe.

In accordance with another aspect of the present invention, theplurality of RF probes (any version) may be used to monitor acorresponding plurality of test wires in the device under test. Forexample, a system containing sixteen probe outputs (or more or less)could be used. Such a system may have sixteen parallel outputs torespective display devices such as a strip chart recorder or may morepreferably include a multiplexor-demultiplexor system that uses a singleoptical link for transmitting the multiplexed information external ofthe shielded enclosure.

Referring now to FIG. 10, a control system for an operator to selectmanually one of a plurality of low cost RF voltage probes 101 to bemonitored, one probe at a time, is shown. In this embodiment, sixteenprobes labeled probes 101a through 101p are illustrated (the lettersuffixes are used to distinguish the otherwise identical probes), eachprobe having a pin 112 for inserting into a test wire 110 of deviceunder test. In this regard, the device under test may be a wire harnessfor use in an automotive vehicle, a circuit having one or more componentparts interconnected, a printed circuit board or combinations of printedcircuit boards and/or device. It is noted that it is not necessary tohave the device under test operational when measuring RF voltage levelsin the wires of the device under test.

Each of the probes 101a-101p is connected to a multiplexor 111 by aconductor 20 in parallel with a resistor 113 passed to ground, e.g., 750KΩ. It is desirable to match the impedance of the probe conductor 20 tothe input of multiplexor 111. In a preferred embodiment, as illustratedin FIGS. 11A and 11B, 12, 12A and 12B, an operational amplifier 117 in avoltage follower configuration and a 1 kΩ resistor 119 may be used.Thus, for a ten foot length of conductor 20, which has an impedance ofabout 250 Kohms, the impedance at the output of the op amplifier isabout zero, thus providing the desired impedance isolation.

Returning to FIG. 10, associated with each conductor 20 is a secondconductor 48 which is used to connect the ground return wire 118 relatedto the wire 110 of the device under test being monitored, to the groundpotential the monitoring circuit instrumentation. In devices under testhaving a single ground return 118, a single conductor 48 may be used toconnect that ground 118 to the ground of the instrumentation. However,when a plurality of ground returns 118 exist in the device under test,as occurs, for example, in an automobile wiring harness, then each wire110 must be monitored with reference to its corresponding ground return118.

In one embodiment, illustrated in FIGS. 12A and 12B, and which may beincorporated into the other embodiments described herein, this may beachieved by including a second multiplexor 111B having a plurality ofinputs for receiving the respective grounds 118a-118p for the wires110a-110p, and an output that is connected to ground potential of theinstrumentation, i.e., the center tap between the two +12 volt batteriesdescribed herein. Each ground wire 118 is coupled to the multiplexer118B input by a corresponding conductor 48a-48p having a pin 128a-128ppassing into wires 118a-118p, and multiplexor 111B is addressed insynchrony with multiplexor 111A with the same input select address sothat the instrumentation ground is always tied to the ground return ofthe circuit under test for the selected probe 101x. In the case thatseveral wires 110 being monitored have the same ground return 118, thenonly one conductor 48 is required, provided that the correspondinginputs of multipelxor 111B are jumpered to the one conductor 48.

Multiplexors 111A and 111B may be a sixteen to one multiplexor having aparallel four bit address input select for controlling the input tooutput connection, and are collectively illustrated as multiplexor 111on FIG. 10. A suitable multiplexor is Model AD7506 available from AnalogDevices which may be used for both multiplexors 111A and 111B, and forthe demultiplexor 193. Other multiplexor devices also could be used.

The system shown in FIG. 10 includes a thumbwheel switch 130, thatcontrols a multiplexor address line including a multi-bit parallel databus 132, a parallel to serial converter 134, a serial data bus 136, adriver amplifier 138, and a light source 140 coupled to an optical fiber142. The foregoing elements are located outside of the shieldedenclosure. The multiplexor control line also includes optical fiber 142passing through the shielded wall into the shielded enclosure andconnecting to a photodetector 144, to a pulse restorer 148 whichprovides a four bit serial output on lead 150, and a serial to parallelencoder 152. The output of the serial to parallel encoder 152 is thenprovided to multiplexor 111 for selecting the input of multiplexor 111.

The output of multiplexor 111 is connected to an RF voltage measurementlink including a voltage to frequency converter 160 on lead 158, whichprovides an output to a driver amplifier 162, which in turn drives alight source 164 coupled to an optical fiber 166. These elements areinside the shielded enclosure. The RF voltage measurement link alsoincluding the optical fiber 166, which passes out of the shieldedenclosure and is coupled to a light receiver 168, which is in turnconnected to a line driver amplifier 170, which is connected to afrequency to voltage converter 172. The output of converter 172 is thenprovided to a display device 174. In addition, the output of switch 130may be connected to display device 180 which includes display circuits182 and a display device 184. In the preferred embodiment, lightemitting diodes (LED) are used to transmit the signals through opticalfibers 142 and 166. Accordingly, the same LEDs may be used for lightsources 164 and 140, the same LED driver amplifiers may be used fordrivers 138 and 162, and the same light detectors 168 and 144 may beused to receive the light transmitted from the LEDS.

In this embodiment, the control switch 130 permits the operator toselect any one of the sixteen probes 101a-101p by appropriate movementof the switch. Thus, the radiated susceptibility test is performednormally and the device under test is evaluated for susceptibility tothe particular frequency and electromagnetic field level wheresusceptibility occurs. Following the radiated susceptibility test, theplurality of probes 101 may be connected to the DUT for the samediagnostic purposes. In this regard, the operator turns the device undertest off and the operator then regenerates the electromagnetic fieldfrequency and voltage level that cause susceptibility during theradiated susceptibility test and selectively monitors each RF probe, oronly those probes of interest, by turning switch 130 to the selectedpositions illustrated 1-16 on FIG. 10. The ability to monitorselectively the probe outputs eliminates the requirement to open andclose the RF enclosure door and moving a single probe 1 as illustratedin FIG. 1, from one wire of the device under test to another wire. Thissubstantially shortens the time required to identify the areas in thedevice under test where susceptibility is occurring and thus shortenssubstantially the time required to locate and cure the problem in thedevice under test.

When the operator selects a specific position on switch 130, that switchprovides a parallel four bit output corresponding to that position. Thefour bits are then converted to serial bits by encoder 134. The serialbits are then transmitted by LED 140 over fiber 142 and received insidethe shielded enclosure by detector 144. The detected signals arerestored as pulses by restorer 148 and converted back into a parallelword by encoder 152. The four bit word provided from switch 130 is thenused to control multiplexor 111 to select the corresponding probe 101x(x corresponding to the switch positions 1-16 (corresponding to lettersa-p)). Output of the probe selected is then provided from the output ofmultiplexor 111. That output voltage, which may be a DC level or anamplitude modulated signal, is then converted to a frequency by voltageto frequency converter 160 which operates driver amplifier 162 tostimulate LED 164 to pass that signal out of the shielded enclosure oncable 166. Voltage to frequency converter 160 typically provides forconverting the DC voltage level on lead 158 to a corresponding frequencyof between 0 and 100 KHz depending on the magnitude of the DC signal. Itis noted that the output DC level is latched and hence remains constantas long as switch 130 remains in the selected position and theexcitation frequency and strength is not changed. Once switch 130 isturned to a different position, the output displayed on meter 174 willchange in response to the 4 bit code corresponding to the desired probe101x.

The passed signal is then detected by detector 168, amplified byamplifier 170, and converted back into a voltage by converter 172.Frequency to voltage converter 172 applies a DC signal to meter 174which is proportional to the DC level at the output of probe 101x. Thevoltage is then displayed on meter 174.

Referring now to FIGS. 11A and 11B, a circuit schematic for themultiplexor system illustrated in FIG. 10 is shown in FIG. 11Aillustrating the portion outside the RF shielded enclosure and FIG. 11Billustrating the portion inside the RF shielded enclosure. In theembodiment, switch 130 is a rotary thumbwheel switch, for example, ModelDRKR16H available from NKK. The switch has 16 positions and a four bitoutput illustrated as bits A1, A2, A3, A4. The position to output bittable is illustrated. Parallel to serial encoder 134 may be a ModelMC145026 available from Motorola. This circuit is configuredsubstantially as illustrated on FIG. 11 to convert the parallel inputfrom switch 130, which input is maintained as long as switch 130 is notchanged, to a serial output. The serial output is then passed throughLED driver 138 which may be a Model 1/2 75451 device available fromMotorola. The device includes a NAND gate and a transistor with theoutput of the transistor being the input to LED 140. The driver circuit138 is connected substantially as shown using +5 volt DC inputs to pins1 and 8, a 100 Ω resistor connected between pins 8 and 3 and pin 4connected to ground. Pin 2 of the device receives the serial output fromencoder 134. Output pin 3 is connected to the anode of light source 140,in the case a light emitting diode Model HFBR1404 available from HewlettPackard. The output of LED 140 is coupled to an optical fiber cable. Thecable passes inside the shielded structure and to a photodetector 144which is a Model HFBR2404 optical receiver, available from HewlettPackard. Pin 6 of device 2404 is connected to a +5 volt DC source and a0.1 microfared capacitor which is grounded. Pins 3 and 7 are tiedtogether and connected to a 1 KΩ potentiometer in series with a 0.1microfared capacitor. The potentiometer is used to drive the LED withthe digital control circuitry. The output of device 2404 is passed topulse restorer 148 which may be a Model 1/4 74LS86 available fromMotorola. This device is a exclusive OR gate having a +5 volt DCreference for restoring the optical pulses to uniform pulse level of 5volts. The output of pulse restorer 148 is passed to serial to parallelencoder 152 which may be Model MC145027, available from Motorola.Encoder 152 converts the serial input from pulse restorer 148 to aparallel output on bus 153. The device MC145027 is configured asillustrated on FIG. 11 in the conventional manner to provide theparallel output. Resistor R1 is 10 Kohms, resistor C1 is 3900picofarads, resistor R2 is 100 Kohms, capacitor C2 is 7500 picofarads,capacitor C3 is 0.1 microfarads. The four bit output is then passed tomultiplexor 111 which is preferably a Model AD7506 available fromMotorola. The output of multiplexor 111 is in turn provided to a voltageto frequency converter 160 which is preferably a Model AD650 availablefrom analog devices. The signal is passed across 100 KΩ resistor intopin 3 of converter 160 and pins 3 and 1 are connected across an inputcapacitor. The output of converter 160, specifically pins 8 and 10 areconnected by a 2.2 KΩ resistor to +5 voltage DC source and passed toground across a capacitor. The output signal is in turn fed across a 1KΩ resistor to the input of driver amplifier 162. Amplifier 162 ispreferably a TTL driver Model DS75451 available from Motorola. Thisdevice is configured in a conventional manner with the +5 voltage sourceconnected across a 1 KΩ resistor to input 1 and connected directly topin 8.

The output of the driver 162 is input to optical transmitter 164,preferably a Model HBFR1404 LED as described. The optical signal isreceived by detector 168, which may be a Model HFBR2404. In thisembodiment, pins 3 and 7 of the device are connected to ground across apotentiometer P1. The value of potentiometer P1 is selected to providefor the device operating into a logic gate such as line driver 170. Linedriver 170 is preferably a 1/4 74LS86 line driver configured the same asdriver 148 previously described. The output of driver 170 is provided toa frequency to voltage converter such as analog devices Model AD650. Theoutput of converter 172 is provided to meter 174 which may then displaythe DC level of the monitored wire of the device under test. In theembodiment illustrated, two +12 volt supplies are used with a center tapcommon ground return. Hence, the reference voltage VDD is +12 volts, thereference voltage VSS is -12 volts, and the signal ground is 0 volts.The +12 volt source also is regulated down to a +5 volt supply for useas illustrated. In all cases, the pin numbering of the devices are thoseprovided by the manufacturers identified, unless otherwise indicated.

Referring now to FIG. 12, an alternate embodiment of the multiplexorsystem is shown. In this case, the same reference numerals used in FIGS.10 and 11 correspond to the same circuit devices and manner ofoperation. In this embodiment, in place of switch 130, a clock 190 andhexcounter 192 are provided. In addition, in place of meter 174, ademultiplexor 193 and a plurality of meters 174a-174p are provided.

In this embodiment, hexcounter 192 is a device having a four bitparallel output, e.g., Model No. MC14520B available from Motorola. Theoutput code is provided to the parallel to serial encoder 134. Clock 190is used to control hexcounter 192 to sequence each of the four bitoutputs to sample each of the probes 101a-101p at the clock rate.Similarly, clock 190 is used to control demultiplexor 192 to display theoutputs of each of probes 101a-101p on the appropriate meter 174a-174pone at a time. Thus, the hexcounter 192, clock 190, and demultiplexor192, and display meters 174a-174p provide for automating the manualoperation described in FIG. 10 and 11. By using a sufficiently fastclock, each of meters 174a-174p will provide apparently simultaneousdisplay of the DC output of each of probes 101a-101p.

Referring to FIGS. 12A and 12B, a preferred embodiment of the presentinvention is shown, in which the same reference numerals used in FIGS.10-12 correspond to the same circuit devices and manner of operation. Inthis embodiment, with reference also to FIG. 12, in place of hex-counter192 and parallel to serial encoder 138, a binary up-counter 194 is used,located outside of the shielded enclosure, and in place of the serial toparallel encoder 152 inside the shielded enclosure, a second binaryup-counter 195 is used. In this embodiment, clock 190 operatesup-counter 194 to count at the clock rate, e.g., 50 KHz. This providesthe sequence of four-bit words that sequentially address demultiplexor193. A line driver amplifier 194a is inserted between clock 190 and upcounter 194. The output of clock 190 also is passed over driver 138 andLED 140, and this is optically transmitted inside the shielded enclosureand recovered by detector 144 and pulse restorer 148. The output ofpulse restorer 148 is thus the clock rate and causes up-counter 195 tocount at the same rate as counter 194. Thus, the two counters are insynchronism and operate both multiplexor 111 (multiplexors 111A and 111Bwhere appropriate) and demultiplexor 193 in synchronism.

In the embodiment illustrated in FIG. 12A and 12B, with the outsidecomponents in FIG. 12A and the inside components in FIG. 12B, frequencyto voltage convertor 172 is located in series with a voltage followeramplifier 171a between each output of multiplexor 193 and each channelof a multichannel display device 174. This parallelism is a designchoice which is an alternative to using a single frequency to voltageconvertor 174 at the input end of multiplexor 193 as shown in FIG. 12.

The center-tapped power supply of two +12 volt rechargable batteries toproduce Vdd (+12 v) and Vss (-12 v) and a five-volt voltage regulatorREG are illustrated in FIG. 12A. Also shown are 1 μf and 1 KΩpotentiometers at the output of the LED receivers 144 and 168, and resetswitches R1 and R2 for manually resetting counters 194 and 195,respectively.

It is to be understood that delay circuits (not shown) may be introducedto provide for maintaining the outputs of counters 194 and 195 in synchwith clock 190. Hence, the outputs of the plurality of probes 101 areautomatically displayed on the corresponding output device 174, e.g., adisplay meter or a strip chart. The voltage to frequency convertorcircuits 160 and 172 may be model AD 650, available from Analog Devices,or model VFC 110. The op amps 117 and 171 may be model AD 843JN,available from Analog Devices. The multiplexors 111A, 111B and 193 maybe model AD 7506 available from Analog Devices or model MPC800 availablefrom Motorola. The up-counters 194 and 195 each may be model MC14520B,available from Motorola. The LEDs 140 and 164 may be model HFBR1404 andthe LED receivers and 168 may be models HFBR2406. One advantage of thisembodiment is that the multiplexor control channel is simplified to onlyproviding a clock rate.

In yet another embodiment, not shown, the multiplexor control channelmay omit any control circuits outside of the shielded enclosure and theoptical transmission elements such that clock 190 is directly connectedto counter 195 for controlling the selection of the probe. In thisembodiment, it is necessary to recover the clock signal from the voltagesignal transmitted over cable 166. This may be done by, for example,interposing a logic circuit that changes state with every clock pulse sothat the change of state can be recovered external to the shieldedenclosure and used to generate the clock for addressing thedemultiplexor 193 in synchrony with multiplexor 111. Other techniquesfor modulating the clock signal onto the data signal for transmissionout of the shielded enclosure may be used.

It should be understood that alternate forms for multiplexing could beused. For example, in place of time multiplexing described in connectionwith FIG. 12, a frequency multiplexing system could be used. In thiscase, each of probes 101a-101p would be connected to an LED driverhaving a different frequency and the frequencies multiplexed onto acommon optical fiber. The common optical fiber is then demultiplexedoutside of the enclosure and separated into the separate frequencysignals. The separate frequency signals can then be demodulated intovoltage levels and separately displayed. Such a circuit may beconstructed from conventionally available parts.

In another embodiment, the output of multiplexor 111 may be transmittedfrom inside to outside the shielded enclosure by using the systemdescribed in copending and commonly assigned U.S. patent applicationSer. No. 862,621 filed Apr. 2, 1992, the disclosure of which is herebyincorporated by reference herein in its entirety.

While illustrative preferred embodiments of the invention have beendisclosed herein, many departures from those embodiments may be madewithout departing from the spirit and scope of the claimed invention,and it is intended that such changes and variations be encompassed, solong as applicant's invention is employed, as defined by the followingclaims.

I claim:
 1. A method for testing the susceptibility of a system toelectromagnetic fields comprising the steps of:a. disposing said systemin a controlled electromagnetic environment, substantially free ofambient electromagnetic fields, b. attaching a probe to said system,said probe including a detector diode having an anode operativelyconnected to the system and a cathode providing a signal representativeof the system at the point of connection, c. connecting said probe to atransmitter, d. connecting a ground of said system to said transmitter,e. producing a controlled electromagnetic field in said controlledelectromagnetic environment, f. detecting, at said probe, the level ofthe field coupled into said system during the production of saidcontrolled electromagnetic field, g. transmitting, to said transmitter,a first transmitted signal related to said detected field level fromsaid probe and a ground return signal from said system, h. connectingsaid transmitter to a monitor and transmitting a second transmittedsignal from said transmitter to said monitor; i. monitoring theamplitude of the second transmitted signal received by the monitor, saidamplitude being at a first value, j. removing the system and the probefrom the controlled electromagnetic environment, k. attaching the probeto the system and connecting the probe and the system ground to themonitor, l. injecting a signal into said system, said injected signalhaving a predetermined relationship to said controlled electromagneticfield, m. detecting, at said probe, the level of the signal injectedinto the system, n. transmitting, to said monitor, a third transmittedsignal related to said detected level of the injected signal, o.monitoring the amplitude of the third transmitted signal received by themonitor, said amplitude being at a second value, p. adjusting, ifnecessary, the amplitude of the injected signal until said second valueof amplitude is approximately equal to said first value of amplitude, q.wherein said steps of injecting and adjusting the amplitude of theinjected signal, outside said controlled electromagnetic environment,substantially simulate the effect of said controlled electromagneticfield on said system in the controlled electromagnetic environment. 2.The method of claim 1, wherein the steps (c) and (d) of connecting saidprobe and ground includes the steps of connecting said probe to thetransmitter and connecting the system ground to the transmitter usingrespective electrically overdamped wires that are substantiallytransparent to electromagnetic fields.
 3. The method of claim 1, whereinthe electrically damped wires are overdamped in a distributed fashion.4. The method of claim 1, wherein the step of producing a controlledelectromagnetic field includes the step of producing a continuous wavefield and the step of detecting, at said probe, the level of the fieldcoupled into the system includes the step of detecting the dc levelsignal in said system.
 5. The method of claim 1, wherein the step oftransmitting the second transmitted signal to the monitor includes thestep of converting the first transmitted signal and ground signal to anoptical signal, transmitting said optical signal through an opticalcable, and re-converting the optical signal to an electrical signalbefore applying the re-converted signal to said monitor.
 6. The methodof claim 1, wherein said step of attaching a probe to said systemincludes the step of attaching a plurality of diodes in series.
 7. Themethod of claim 1, wherein said step of injecting a signal includes thestep of direct voltage injection of said signal into said system.
 8. Themethod of claim 7, wherein said step of direct voltage injectionincludes the step of applying the output of a signal generator to saidsystem by way of a plurality of parallel connected capacitors.
 9. Themethod of claim 1, further comprising the steps of applying a correctivedevice to said system;re-injecting a signal into said system at theamplitude determined in step (o); and determining whether the correctivedevice has substantially eliminated the susceptibility of the system tosaid controlled electromagnetic field.
 10. A method for use in testing acircuit for electromagnetic susceptibility comprising the steps of:(a)connecting a probe having a detector diode to a wire in the circuitunder test and monitoring a first signal produced in the circuit whilethe circuit is in a radiation field, the radiation field having aplurality of frequencies, and thereafter (b) monitoring the first signalwhile the circuit is in the radiation field at the plurality offrequencies; and (c) injecting a second signal into the circuit whilethe circuit is no longer in the radiation field to simulate the effectof the radiation field on the circuit.
 11. The method of claim 10wherein the radiation field further comprises a plurality of frequencieshaving a continuous wave signal.
 12. The method of claim 10 wherein step(a) further comprises monitoring the first signal using a probe havingmore than one detector diode in series.